3i. Stormwater Management and Infrastructure

Where does all that rainwater go?

This map shows where stormwater goes in the city and where green infrastructure improvements (tree planting, green streets, ecoroofs, stream restoration) could make the most difference by removing pollutants, reducing runoff or improving natural conditions. The darker the colors, the more benefits we can expect.

The City of Portland manages stormwater in one of these ways:

Older Portland neighborhoods have a combined sewer system, which collects stormwater runoff from streets and sewage from buildings in the same pipes. Most of this mixture flows to the treatment plant. But when it rains, some combined sewage overflows to the Willamette River. When Portland’s CSO control program is complete at the end of 2011, combined sewers will overflow an average of three times each winter and once every three summers instead of every time it rains.

In much of Portland, either east of 52nd Avenue or north of Fremont Street, stormwater runoff flows into the ground via sumps, also known as underground injection control (UIC) facilities. Sumps are perforated, vertical pipes usually connected to sedimentation manholes which help remove pollutants. Sedimentation manholes collect stormwater, allowing solids to settle to the bottom and trapping oils and greases in the manhole before the treated stormwater flows to the sump and percolates into the ground. In areas where groundwater is high, the city is redesigning some UICs and replacing others with swales and other green stormwater management facilities to increase the distance between the bottom of the UIC and groundwater. These types of facilities replenish groundwater supplies that feed cool, clean water to rivers and streams.

The rest of the city has a separate storm sewer system. Sanitary sewers carry sewage from buildings to the treatment plant, and stormwater runoff flows to streams through public and private pipes, drainages, swales and other stormwater conveyances. Stormwater runoff that isn’t properly managed can cause stream bank erosion, landslides and flooding, and harm water quality.

In all of these areas, the City of Portland invests in stormwater management projects that mimic nature and manage stormwater at its source. Swales, rain gardens and other green stormwater management facilities reduce stormwater runoff, and filter sediments and other pollutants. These projects also help the city meet federal Clean Water Act regulations aimed at restoring and maintaining the health of rivers, streams, lakes, and wetlands.

In some areas, redevelopment can improve environmental conditions by replacing parking areas, paving, conventional roofs and other impervious areas with planted stormwater facilities like green streets, swales or ecoroofs. These facilities slow and reduce stormwater runoff. Trees and other vegetation can also beautify, shade and cool the site and provide habitat for native birds and pollinators.